Optical refrigeration can include laser excitation of rare-earth doped glass and crystal host material systems. In some systems, optical refrigeration can include use of ytterbium doped fluorozirconate glass (Yb:ZBLAN).
In an optical refrigeration system, a cooling cycle is based on conversion of low-entropy low-energy input of an optical field (e.g. laser) into an isotropic higher-energy spontaneous emission (fluorescence). The excitation laser is red-shifted from a mean wavelength of the emitted fluorescence (λf). Following absorption, out of equilibrium excitation becomes thermalized within the ground and exited state manifolds of the rare-earth ion. This is accomplished by phonon absorption from lattice vibrations of a material host. Thermal quanta of energy kT are carried away from the host in a form of spontaneously emitted photons, thereby cooling the material.
Heat generating, non-radiative recombination pathways are undesirable, leading to a demand for high quantum efficiency (ηeqe) materials. Rare-earth ions (e.g. Ytterbium) exhibit metastable transitions in the lowest energy levels, satisfying a requirement for high quantum efficiency. An additional requirement is for high host purity. Impurities (e.g. transition-metals) introduce extrinsic heat generating recombination pathways, which are manifested in reduction of absorption efficiency, ηabs, further defined below. Both of these requirements for high quantum efficiency and absorption efficiency are captured in an expression for cooling efficiency, defined as a ratio of cooling power to absorbed power:
                                          η            c                    ⁡                      (                          λ              ,              T                        )                          =                                            P              cool                                      P              abs                                =                                                                      η                  eqe                                ⁡                                  [                                      1                                          1                      +                                                                        α                          b                                                /                                                                              α                            r                                                    ⁡                                                      (                                                          λ                              ,                              T                                                        )                                                                                                                                ]                                            ⁢                                                          ⁢                              λ                                                      λ                    f                                    ⁡                                      (                    T                    )                                                                        -            1                                              (        1        )            where λ denotes laser wavelength, αb is the background absorption coefficient, and αr(λ) is the resonant absorption coefficient of Yb ions. The product in brackets is denoted as absorption efficiency ηabs. In practical applications, material sheds a kBT of thermal energy per excitation, which means that the product ηeqeηabs>1−kBT/hνF, where νF=c/λF, and c=speed of light. For example, for room temperature Yb emission, this product must be larger than 96.8%, while at a temperature of 100K, this product must be larger than 99%. Because ηeqe is governed by intrinsic recombination mechanisms for a given material, ηabs must be improved in order to obtain low temperature operation of an optical refrigerator.
As material cools, resonant absorption in the anti-Stokes tail falls exponentially, following the Boltzmann law. This is the main reason which halts cooling at low temperatures. In order to achieve cooling to lower temperatures requirement of high absorption efficiency has to be met. In order to increase the absorption efficiency (ηabs) one has to minimize the ratio αb/αr(λ). A common approach to accomplish that has been to lower background absorption by purifying a material growth process and starting materials. A more recent method has relied on an increase of the resonant absorption (together with low αb) to enhance the absorption efficiency.
These challenges can be been dealt with herein, using Stark manifold resonances to increase cooling efficiency in optical refrigeration devices, as will be described in connection with the exemplary embodiments that follow.